Let's break this down. We're going to add 251 and 6405 together.

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

Step 2: We'll start by adding the digits 5 & 0 in column 2 and get 5.

Step 3: We'll start by adding the digits 2 & 4 in column 3 and get 6.

Step 4: We'll start by adding the digits 0 & 6 in column 4 and get 6.

So, 251 + 6405 = 6656.

